The present invention relates to a block polymer, a process for its production and a liquid composition containing the block polymer, as well as a polymer electrolyte fuel cell having an electrode containing the block polymer.
Attention has been drawn to a hydrogen/oxygen fuel cell as a power generation system which gives substantially no adverse effect to the global environment, since its reaction product is only water in principle. Polymer electrolyte fuel cells were once mounted on space ships in the Gemini Project and the Bio-satellite Project, but the cell output densities at that time were low. Thereafter, alkaline fuel cells having higher performance have been developed, and such alkaline fuel cells have been employed for space crafts including current space shuttles.
Whereas, along with the progress of technology in recent years, attention has been drawn again to polymer electrolyte fuel cells, for the following two reasons. (1) As a polymer electrolyte, a highly conductive membrane has been developed. (2) A catalyst to be used for a gas diffusion electrode layer is supported on carbon, and it is further coated with an ion exchange resin, whereby it has been made possible to obtain an extremely large activity.
And, many studies are being made on a process for producing a polymer electrolyte membrane/electrode assembly (hereinafter referred to simply as the assembly) for a polymer electrolyte fuel cell.
Polymer electrolyte fuel cells which are presently being studied, have an operation temperature as low as from 50 to 120xc2x0 C. and thus have a drawback that waste heat can hardly be utilized, for example, as an auxiliary power for fuel cells. To offset such a drawback, it is desired that the polymer electrolyte fuel cells have particularly high output densities. Further, for practical applications, it is desired to develop an assembly whereby a high energy efficiency and a high output density can be obtained even under an operation condition where the fuel and air utilization rates are high.
Under an operation condition where the operation temperature is low and the gas utilization rate is high, clogging (flooding) of the electrode pore is likely to take place due to condensation of water-vapor at the cathode where water is formed by a cell reaction. Accordingly, in order to obtain a stable performance for a long period of time, it is necessary to secure water repellency of the electrode so as to prevent such flooding. This is particularly important in the case of a polymer electrolyte fuel cell whereby a high output density can be obtained at a low temperature.
In order to secure water repellency of an electrode, it is effective to reduce the ion exchange capacity of the ion exchange resin covering the catalyst in the electrode, i.e. to use an ion exchange resin having a low content of ion exchange groups. However, in such a case, the ion exchange resin has a low water content, whereby the electrical conductivity tends to be low, and the cell performance tends to be low. Further, the gas permeability of the ion exchange resin decreases, whereby supply of the gas to the catalyst surface through the coated ion exchange resin, tends to be slow. Accordingly, the gas concentration at the reaction site tends to be low, whereby the voltage loss tends to be large, i.e. the concentration overpotential tends to be high, whereby the output tends to decrease.
Therefore, it has been attempted to use a resin having a high ion exchange capacity as the ion exchange resin to cover the catalyst and further to incorporate to an electrode, particularly to a cathode, a fluorine resin such as polytetrafluoroethylene (hereinafter referred to as PTFE), a tetrafluoroethylene (hereinafter referred to as TFE)/hexafluoropropylene (hereinafter referred to as HFP) copolymer or a TFE/perfluoro(alkyl vinyl ether) copolymer, as a water repellent, thereby to suppress flooding (JP-A-5-36418). In this specification, an A/B copolymer represents a copolymer comprising repeating units based on A and repeating units based on B.
However, if the amount of the above water repellant in the electrode is increased in order to provide adequate water repellency, the electrical resistance of the electrode will increase as the above water repellent is an insulating material. Further, as the thickness of the electrode increases, the gas permeability decreases, thus leading to a problem that the output will decrease. To complement the decrease in the electrical conductivity of the electrode, it is, for example, necessary to increase the electrical conductivity of the carbon material as the carrier of the catalyst or to increase the ionic conductivity of the ion exchange resin covering the catalyst. However, it is difficult to obtain an electrode which satisfies adequate electrical conductivity and adequate water repellency at the same time, and it has not been easy to obtain a polymer electrolyte fuel cell which provides high output and which is stable for a long period of time.
Further, a method of mixing fluorinated pitch (JP-A-7-211324) or a method of subjecting a catalyst carrier to fluorination treatment (JP-A-7-192738) has also been proposed, but there has been a problem that the catalyst surface can not be uniformly covered by an ion exchange resin. Further, a method of letting the water repellency have a gradient in the thickness direction of the electrode, has been proposed (JP-A-5-251086, JP-A-7-134993), but the production method is cumbersome.
In order to solve such problems, the present inventors have studied the possibility of using for an electrode an ion exchange resin comprising segments containing sulfonic acid groups (xe2x80x94SO3H groups) and fluorine-containing segments having substantially no ion exchange groups. As one of such polymers, a block polymer may be mentioned, and as a method for its production, a method of employing iodine transfer polymerization is disclosed in JP-B-58-4728. Namely, disclosed is a method of polymerizing at least two radical-polymerizable monomers having unsaturated bonds by radical polymerization in the presence of an iodine compound having iodine atom(s), wherein in order to let at least two types of polymer chain segments form between the carbon-iodine bond of the above-mentioned iodine compound, the monomers to form the above-mentioned respective polymer chain segments, are sequentially polymerized to obtain a multi-segmented polymer.
And, as the monomers to form such segments, TFE and a monomer of the formula CF2xe2x95x90CFYxe2x80x2 (wherein Yxe2x80x2 is represented by xe2x80x94(OCF2)xcex1xe2x80x94(OCF2CF2)xcex2xe2x80x94(OCF2CF(CF3)xcex3xe2x80x94Z, Z is SO2F or SO3Mxe2x80x2, Mxe2x80x2 is a hydrogen atom, a sodium atom or a potassium atom, and each of xcex1, xcex2 and xcex3 is an integer of from 0 to 3, provided that xcex1+xcex2+xcex3 greater than 0) are disclosed, and CF2xe2x95x90CFOCF2CF(CF3)OCF2CF2SO2F is also exemplified. Further, WO98/43952 discloses that in the presence of I(CF2)4I, CF2xe2x95x90CF2/CF2xe2x95x90CFOCF2CF2SO3Na copolymer segments (59/41 in molar ratio) are prepared by polymerization and then CF2xe2x95x90CF2/CF2xe2x95x90CFOC3F7 segments are synthesized, to obtain a block polymer of ABA type having a heat absorption peak (a crystal melting point) at 275xc2x0 C. as measured by a differential scanning calorimeter (DSC).
Such a polymer is disclosed to be excellent in e.g. electrical resistance or water resistance. However, the specifically disclosed block polymer has crystallinity, and it is difficult to control the polymerization, and such a block polymer can hardly be dissolved or dispersed in a solvent. Accordingly, in its application as a material for an electrode for a fuel cell, such a block polymer has a problem that it can hardly be formed into an electrode.
In order to increase the output of a fuel cell, the ion exchange resin in the electrode is required to be highly gas permeable and highly conductive, and an ion exchange resin having a high ion exchange group concentration and a high water content, is preferred. However, if an ion exchange resin having a high ion exchange group concentration is employed, flooding is likely to take place and the output tends to decrease when it is used for a long period of time, although the fuel gas permeability and conductivity will be high and the initial output of the fuel cell will be high.
Accordingly, it is an object of the present invention to provide a block copolymer which is useful as an electrode material for a fuel cell and which has a high water content and water repellency, a process for its production and a liquid composition containing such a block polymer, as well as a polymer electrolyte fuel cell employing such a block polymer.
The present invention provides a polymer electrolyte fuel cell comprising a membrane-form polymer electrolyte, a cathode disposed on one side of the polymer electrolyte, and an anode disposed on the other side of the polymer electrolyte, wherein the cathode comprises a catalyst and an ion exchange resin, and the ion exchange resin is made of a polymer comprising the following segments A and segments B:
Segments A: segments made of a polymer having sulfonic acid groups; and
Segments B: segments made of a fluoropolymer having substantially no ion exchange groups.
Further, the present invention provides a block polymer comprising the following segments Cxe2x80x2 and the following segments D and having a molecular weight of from 5xc3x97103 to 5xc3x97106.
The present invention also provides a block polymer comprising the following segments Cxe2x80x2 and the following segments E and having a molecular weight of from 5xc3x97103 to 5xc3x97106.
Segments Cxe2x80x2: segments made of a copolymer comprising repeating units based on a perfluorovinyl ether having a xe2x80x94SO2X group (wherein X is a fluorine atom, a chlorine atom or OM wherein M is a hydrogen atom, an alkali metal atom or NR1R2R3R4, wherein each of R1, R2, R3 and R4 which are independent of one another, is a hydrogen atom or a monovalent organic group) and repeating units based on tetrafluoroethylene, wherein the repeating units based on the perfluorovinyl ether are contained in an amount of at least 20 mol %;
Segments D: segments made of an amorphous perfluoropolymer having no ion exchange groups and having alicyclic structures in its main chain; and
Segments E: segments made of an amorphous copolymer having no ion exchange groups and comprising repeating units based on a perfluoro(alkyl vinyl ether) and repeating units based on tetrafluoroethylene, wherein the repeating units based on the perfluoro(alkyl vinyl ether) are contained in an amount of at least 20 mol %.
Further, the present invention provides a liquid composition which comprises an organic solvent having an alcoholic hydroxyl group and a block copolymer comprising the above-mentioned segments Cxe2x80x2 and the above-mentioned segments D, or a block copolymer comprising the above segments Cxe2x80x2 and the above segments E, dissolved or dispersed in the organic solvent.
Still further, the present invention provides a process for producing a polymer, which comprises polymerizing a first monomer group comprising one or more fluoromonomers in the presence of a first radical initiator and an iodine-containing fluoro-compound in a non-aqueous medium or in the absence of any solvent, to prepare a substantially amorphous polymer, and then, polymerizing a second monomer group comprising one or more monomers different from the first monomer group in the presence of the polymer and a second radical initiator in a non-aqueous medium or in the absence of any solvent.